The purpose of this research is to contribute to our understanding of the neuronal machinery responsible for learning and response selection in complex, integrated behavioral repertoires. These related problems are to be studied in a relatively simple system, the lateral giant escape reaction circuit of the crayfish, which is subject to a variety of forms of control and modulation. Behavioral and electrophysiological research is proposed on (1) the neural organization and pharmacology of a descending tonic inhibitory system that modulates lateral giant escape excitability and is involved in habituation and perhaps associative learning of escape as well as suppression of escape during competing activities, (2) the possible role of tonic inhibition in reprogramming lower level circuitry so that the intrinsic excitability of escape is readjusted, (3) the possible role of transmitter auto-receptors in inducing habituation, and (4) the neurochemical mediators of sensitization learning. Computational research is proposed on the selection pressures that resulted in the existence of pattern-specific recognition neurons and response-dedicated trigger neurons in this system with a consideration of the possible generality of these selection pressures.